Why does Tom get revenge on Casy's murder when he knows it will hurt his family?

Answers
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Casy, who sacrificed his freedom for Tom earlier in the novel, makes a final sacrifice in this chapter, the victim of a brutal murder at the hands of the police. Casy has now been a martyr for both the Joad family and the entire class that the Joads represent. Tom reacts out of loyalty and friendship. He believes he's helping Casy and never thought the fight would end in murder.

The effect of this martyrdom is that Tom must now leave Hooper ranch to escape capture by the police. Although he wishes to go alone, Ma Joad once again binds the Joad family together. She chooses to risk the safety of the entire family to preserve whatever unity the family has left.